Semiconductor device

ABSTRACT

A semiconductor device including a substrate and a shallow trench isolation (STI) structure is provided. The substrate has a first voltage area and a second voltage area. A top surface of the substrate in the second voltage area is higher than a top surface of the substrate in the first voltage area, and a trench is defined in the substrate in between the first and second voltage area. The STI structure is located in the substrate within the trench, wherein a first portion of the STI structure is located in the first voltage area, a second portion of the STI structure is located in the second voltage area, and a step height difference exist in between a bottom surface of the first portion of the STI structure in the first voltage area and a bottom surface of the second portion of the STI structure in the second voltage area.

CROSS-REFERENCE TO RELATED APPLICATION

This is a continuation application of and claims the priority benefit of U.S. application Ser. No. 15/287,535, filed on Oct. 6, 2016, now allowed. The entirety of the above-mentioned patent application is hereby incorporated by reference herein and made a part of this specification.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention generally relates to a semiconductor device, in particular, to a semiconductor device having different voltage areas, and a method of forming the same.

2. Description of Related Art

For MOS structures having metal gate, high K materials are commonly used as the gate oxide material for increasing gate capacitance. During the processing of such MOS structure, a thicker gate oxide is generally observed in higher voltage areas. Due to the thicker gate oxide layers, a gate height in the high voltage area is also higher than a gate height in the low voltage area. As such, the gate height in the higher voltage area may be lost during a subsequent chemical-mechanical planarization (CMP) process. Furthermore, a height difference may also be observed in the shallow trench isolation (STI) structure, and it is possible that during the formation of the gate structure, polysilicon residues may remain in lower height areas of the STI structure. As a result, the height differences observed in the gate and STI structures may cause problems such as device mismatch and reduced performance. Based on the above, there is a need to provide a method that minimize the height differences while still maintaining the variation in the gate oxide thickness in different voltage areas.

SUMMARY OF THE INVENTION

Accordingly, the present invention is directed to a semiconductor device and a method of forming the same, whereby the gate height differences is minimized and the variation in the gate oxide thickness is maintained in different voltage areas.

The invention provides a method of forming a semiconductor device including the following steps. A substrate having a first voltage area and a second voltage area is provided. A first oxide layer is formed in the first voltage area, wherein the first oxide layer grows up and down from a top surface of the substrate in the first voltage area. The first oxide layer is removed to form a recess in the first voltage area. A shallow trench isolation (STI) structure is formed in the substrate, wherein a first portion of the STI structure is located in the first voltage area and a second portion of the STI structure is located in the second voltage area, a top surface of the STI structure is higher than the top surface of the substrate, and a bottom surface of the first portion of the STI structure in the first voltage area is lower than a bottom surface of the second portion of the STI structure in the second voltage area.

In an embodiment of the invention, prior to the formation of the first oxide layer, a first mask layer is formed on the substrate in the second voltage area, and a thermal oxidation process is used to form the first oxide layer in the first voltage area.

In an embodiment of the invention, prior to the formation of the STI structure, a second mask layer is formed on the substrate in parts of the first voltage area and the second voltage area, and the substrate not protected by the second mask layer is etched to form a trench such that the STI structure is formed within the trench.

In an embodiment of the invention, after the formation of the STI structure, the following steps are performed. The first portion of the STI structure in the first voltage area is partially removed. A second oxide layer is formed on the first voltage area while a third oxide layer is formed on the second voltage area, wherein the second oxide layer grows up and down from the top surface of the substrate in the first voltage area, and the third oxide layer grows up and down from the top surface of the substrate in the second voltage area. The third oxide layer is removed and the second portion of the STI structure located in the second voltage area is partially removed, such that the top surface of the STI structure, the top surface of the second oxide layer and the top surface of the substrate in the second voltage area are substantially coplanar to generate a coplanar surface.

In an embodiment of the invention, a bottom surface of the second oxide layer is located below the coplanar surface.

In an embodiment of the invention, after the formation of the STI structure and prior to the partial removal of the first portion of the STI structure in the first voltage area, the second portion of the STI structure in the second voltage area is covered with a third mask layer, and the first portion of the STI structure in the first voltage area is partially removed by wet etching.

In an embodiment of the invention, prior to the removal of the third oxide layer and the partial removal of the second portion of the STI structure, the second oxide layer and the first portion of the STI structure in the first voltage area is covered with a fourth mask layer, and the removal of the third oxide layer and the partial removal of the second portion of the STI structure is then performed by wet etching.

In an embodiment of the invention, the recess has a recess depth H1, and a step height difference H2 exist between the bottom surface of the first portion of the STI structure in the first voltage area and the bottom surface of the second portion of the STI structure in the second voltage area, wherein H1=H2.

In an embodiment of the invention, the operating voltage of the first voltage area is from 6V to 8V.

In an embodiment of the invention, the operating voltage of the second voltage area is from 0.9V to 1V.

The invention further provides a semiconductor device including a substrate, a shallow trench isolation (STI) structure. The substrate has a first voltage area and a second voltage area. The STI structure is located in the substrate, wherein a first portion of the STI structure is located in the first voltage area, a second portion of the STI structure is located in the second voltage area and a bottom surface of the first portion of the STI structure in the first voltage area is lower than a bottom surface of the second portion of the STI structure in the second voltage area.

In an embodiment of the invention, the semiconductor device further comprises a first gate oxide layer, a second gate oxide layer and a first gate structure. The first gate oxide layer is located on the substrate in the first voltage area, wherein a top surface of the STI structure, a top surface of the first gate oxide layer and a top surface of the substrate in the second voltage area are substantially coplanar to generate a coplanar surface. The second gate oxide layer is located on the first gate oxide layer. The first gate structure is located on the second gate oxide layer in the first voltage area.

In an embodiment of the invention, the semiconductor device further comprises a second gate oxide layer and a second gate structure. The second gate oxide layer is located on the substrate in the second voltage area. The second gate structure is located on the second gate oxide layer in the second voltage area.

In an embodiment of the invention, a bottom surface of the first gate oxide layer is located below the coplanar surface.

In an embodiment of the invention, the operating voltage of the first voltage area is from 6V to 8V.

In an embodiment of the invention, the operating voltage of the second voltage area is from 0.9V to 1V.

In an embodiment of the invention, a material of the second gate oxide layer is HfO₂.

Based on the above, by forming a recess in the first voltage area, the gate height differences between different voltage areas can be appropriately tuned in the subsequent processes, such that the top surface of the STI structure, the top surface of the second oxide layer and the top surface of the substrate in the second voltage area are substantially coplanar, and the gate structures formed thereon will have approximately the same height. As such, the problem of device mismatch and reduced performance can be resolved.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention.

FIG. 1A to FIG. 1K is the process flow of forming a semiconductor device according to an embodiment of the invention.

DESCRIPTION OF THE EMBODIMENTS

Reference will now be made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the description to refer to the same or like parts.

FIG. 1A to FIG. 1K is the process flow of forming a semiconductor device according to an embodiment of the invention. Referring to FIG. 1A, a substrate 102 having a first voltage area 102A and a second voltage area 102B is provided. In an embodiment of the invention, the substrate 102 is a silicon substrate suitable for forming devices with different voltages thereon. For example, devices having a lower operating voltage, such as 0.9V or 1V may be formed on one side of the silicon substrate 102, while devices having higher operating voltage, such as 6V or 8V may be formed on another side of the silicon substrate 102. In the present embodiment, the operating voltage of the first voltage area 102A may be from 6V to 8V. On the other hand, the operating voltage of the second voltage area 102B may be from 0.9V to 1 V.

As shown in FIG. 1A, a first mask layer 104 is formed on the substrate 102 in the second voltage area 102B. In an embodiment of the invention, the first mask layer 104 consist of a photoresist layer 104A, a hard mask layer 104B and a buffer layer 104C stacked sequentially on the substrate 102. In the present embodiment, the hard mask layer 104B is a silicon nitride hard mask layer, while the buffer layer 104C is an oxide layer which serve as the buffer layer of the silicon nitride hard mask layer. The first mask layer 104 only covers the second voltage area 102B, while the first voltage area 102A is exposed.

In a subsequent step as shown in FIG. 1B, a first oxide layer OL1 is formed in the first voltage area 102A, wherein the first oxide layer OL1 grows up and down from a top surface 102T of the substrate 102 in the first voltage area 102A. In the present embodiment, a thermal oxidation process is used to form the first oxide layer OL1 in the first voltage area 102A. The thickness of the first oxide layer OL1 may be appropriately controlled and selected as the thickness of the first oxide layer OL1 directly affects the depth of a recess formed in the first voltage area 102A in a subsequent step.

Next, as shown in FIG. 1C, the first oxide layer OL1 is removed to form a recess in the first voltage area 102A, wherein the recess has a recess depth H1. In the present embodiment, the first oxide layer OL1 is completely removed by wet etching, and the first mask layer 104 covering the second voltage area 102B is removed thereafter. As such, a step height difference is generated between the first voltage area 102A and the second voltage area 102B, wherein the step height difference corresponds to the recess depth H1. As mentioned above, the recess depth H1 is directly affected by the thickness of the first oxide layer OL1. More specifically, the thickness of the first oxide layer OL1 that grows below the top surface 102T of the substrate 102 will correspond to the recess depth H1. That is, if a greater recess depth H1 is desired, then a thicker oxide layer OL1 that grows up and down from the top surface 102T of the substrate 102 will be required.

After formation of the recess, a shallow trench isolation (STI) structure 200 is formed in the substrate 102 as shown in FIG. 1D. Specifically, prior to the formation of the STI structure 200, a second mask layer 106 is formed on the substrate 102 in parts of the first voltage area 102A and the second voltage area 102B, while other parts of the of the first voltage area 102A and the second voltage area 102B are exposed for forming the STI structure 200. The second mask layer 106 consists of a hard mask layer 106A and a buffer layer 106B stacked sequentially on the substrate 102. The hard mask layer 104B is a silicon nitride hard mask layer, while the buffer layer 104C is an oxide layer which serves as the buffer layer of the silicon nitride hard mask layer. In the present embodiment, the substrate 102 not protected by the second mask layer 106 is etched to form a trench TR. The trench TR is formed to have a step height difference H2 at the bottom surface of the trench. More specifically, the step height H2 is generated due to the presence of the recess depth H1 in the substrate 102. That is, the step height difference H2 and the recess depth H1 will correspondingly have the same height.

Thereafter, an isolation material, such as silicon oxide material, is filled into the trench TR to form the STI structure 200. Excess isolation material may be removed by using the chemical-mechanical planarization (CMP) process until the top surface of the second mask layer 106 is exposed.

As shown in FIG. 1D, for the formed STI structure 200, a first portion 200A of the STI structure 200 is located in the first voltage area 102A and a second portion 200B of the STI structure 200 is located in the second voltage area 102B. Furthermore, a top surface TS of the STI structure 200 is higher than the top surface 102T of the substrate. In addition, due to the presence of the step height difference H2 formed at the bottom surface of the trench TR, the STI structure 200 located within the trench TR will have a structure where a bottom surface BS1 of the first portion 200A of the STI structure 200 in the first voltage area 102A is lower than a bottom surface BS2 of the second portion 200B of the STI structure 200 in the second voltage area 102B. That is, in the present embodiment, the recess has a recess depth H1, and a step height difference H2 exist between the bottom surface BS1 of the first portion 200A of the STI structure 200 in the first voltage area 102A and the bottom surface BS2 of the second portion 200B of the STI structure 200 in the second voltage area 102B, wherein H1=H2.

Next, as shown in FIG. 1E, the first portion 200A of the STI structure 200 in the first voltage area 102A is partially removed so as to generate a height difference H3 at the top of the STI structure 200. More specifically, prior to the partial removal of the first portion 200A of the STI structure 200 in the first voltage area 102A, the second portion 200B of the STI structure 200 in the second voltage area 102B is covered with a third mask layer 108, such as a photoresist layer. Thereafter, the first portion 200A of the STI structure 200 in the first voltage area 102A is partially removed by wet etching so as to generate the height difference H3.

Next, as shown in FIG. 1F, the third mask layer 108 and the hard mask layer 106A are first removed. In the present embodiment, the remaining buffer layer 106B and the STI structure 200 is made of oxides, therefore, they can be etched off at the same time. More specifically, an etching back process is performed to completely remove the buffer layer 106B, while a height of the STI structure 200 is further reduced during etching. As such, a height difference between the top surfaces TS of the STI structure 200 and the top surfaces 102T of the substrate 102 is reduced and controlled.

Overall, a height difference (recess depth H1) can be observed between the top surfaces 102T of the first voltage area 102A and the second voltage area 102B, while a height difference H3 can be observed between the top surfaces TS of the first portion 200A and the second portion 200B of the STI structure 200. Furthermore, the height of the STI structure 200 is controlled during etching such that it still protrudes out from the top surfaces 102T of the substrate 102.

In the step shown in FIG. 1G, a second oxide layer OL2 is formed in the first voltage 102A while a third oxide layer OL3 is formed in the second voltage area 102B. In the present embodiment, the second oxide layer OL2 grows up and down from the top surface 102T of the substrate 102 in the first voltage area 102A, and the third oxide layer OL3 grows up and down from the top surface 102T of the substrate 102 in the second voltage area 102B. More specifically, the second oxide layer OL2 that grows up from the top surface 102T of the substrate 102 will become coplanar with the top surface TS of the first portion 200A of the STI structure 200. Furthermore, the third oxide layer OL3 that grows below the top surface 102T of the substrate 102 will become coplanar with the top surface TS of the first portion 200A.

To achieve such coplanar structure, the recess depth H1, the height difference H3 and the etching back process of the STI structure 200 shown in FIG. 1F may be appropriately tuned and calculated such that the second oxide layer OL2 will grow until it matches the height of the first portion 200A of the STI structure 200 in the first voltage area 102A.

Next, in the step shown in FIG. 1H, the second oxide layer OL2 and the first portion 200A of the STI structure 200 in the first voltage area 102A are covered with a fourth mask layer 110. The fourth mask layer 110 only covers the first voltage area 102A, whereas the third oxide layer OL3 and the second portion 200B of the STI structure 200 are exposed.

Subsequently, as shown in step 1I, the removal of the third oxide layer OL3 and the partial removal of the second portion 200B of the STI structure 200 is then performed by wet etching. In the present embodiment, after removal of the third oxide layer OL3 and the partial removal of the second portion 200B of the STI structure 200, the top surface 102T of the substrate 102 and the top surface TS of the second portion 200B of the STI structure 200 in the second voltage area 102 b will become coplanar. More specifically, prior to etching, although the third oxide layer OL3 and the second portion 200B of the STI structure 200 are not at equal height, the two layers may be etched to become coplanar. That is, etching is performed to completely remove the third oxide layer OL3, while the second portion 200B of the STI structure 200 maybe continuously etched until it becomes coplanar with the top surface 102T of the substrate 102.

Next, as shown in FIG. 1J, upon completion of the wet etching step, the fourth mask layer 110 will be removed to expose the surfaces of the substrate 102, the STI structure 200 and the second oxide layer OL2. At this point, it is possible to confirm that the top surface 102T of the substrate 102 in the second voltage area 102B is coplanar with the top surface OLT of the second oxide layer OL2. In addition, it is also possible to confirm that the top surface TS of the first portion 200A of the STI structure 200 is coplanar with the top surface TS of the second portion 200B of the STI structure 200. As such, the top surface TS of the STI structure 200, the top surface OLT of the second oxide layer OL2 and the top surface 102T of the substrate 102 in the second voltage area 102B are substantially coplanar to generate a coplanar surface (102T/TS/OLT being coplanar).

In the present embodiment, the second oxide layer OL2 may be treated as a first gate oxide layer GO1 of the first voltage area 102A. The purpose of having the first gate oxide layer GO1 is so that it compensates for the thicker gate oxide layer required in the first voltage area 102A. In addition, a bottom surface of the second oxide layer OL2 (or the first gate oxide layer GO1) is located below the coplanar surface. That is, the second oxide layer OL2 (or the first gate oxide layer GO1) may be considered as being embedded inside the substrate 102 and below the coplanar surface.

Finally, as shown in FIG. 1K, a semiconductor device may be achieved by further disposing a second gate oxide layer GO2, a first gate structure 310 and a second gate structure 320 thereon. In the present embodiment, the first gate structure 310 consist of a polysilicon layer 310A and a hard mask layer made of an oxide layer 310B and a nitride layer 310C. Similarly, the second gate structure 320 consist of a polysilicon layer 320A and a hard mask layer made of an oxide layer 320B and a nitride layer 320C. Furthermore, in the present embodiment, a material of the second gate oxide layer GO2 is HfO₂, which is a high-κ dielectric material. However, the material of the gate oxide layers is not particularly limited thereto, and any other suitable gate oxide materials may be selected.

As shown in the steps above, the first gate oxide layer GO1 is pre-formed and located on the substrate 102 in the first voltage area 102A. The second gate oxide layer GO2 is then formed on the substrate 102 in the second voltage area 102B and on the first gate oxide layer GO1 in the first voltage area 102A. As such, the first gate structure 310 may be formed on the second gate oxide layer GO2 in the first voltage area 102A, and the second gate structure 320 may be formed on the second gate oxide layer GO2 in the second voltage area 102B. Since the top surface TS of the STI structure 200, the top surface OLT of the second oxide layer OL2 and the top surface 102T of the substrate 102 in the second voltage area 102B are substantially coplanar, the first gate structure 310 and the second gate structure 320 disposed thereon can be controlled to have the same gate height. As such, the problem of device mismatch and reduced performance can be resolved.

Accordingly, by forming a recess in the first voltage area having a recess depth H1 and forming a STI structure to generate a height difference H3, and controlling the etching back process of the STI structure, the gate height differences between different voltage areas can be appropriately tuned in the subsequent processes. Therefore, the top surface of the STI structure, the top surface of the second oxide layer and the top surface of the substrate in the second voltage area are substantially coplanar to generate a coplanar surface, and the gate structures formed thereon will have approximately the same height. As such, the semiconductor device of the invention can have better performance, and the problem of device mismatch can be resolved.

It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention cover modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents. 

What is claimed is:
 1. A semiconductor device, comprising: a substrate, having a first voltage area and a second voltage area; a shallow trench isolation (STI) structure located in the substrate in between the first voltage area and the second voltage area, wherein a first portion of the STI structure is located in the first voltage area, a second portion of the STI structure is located in the second voltage area, and a step height difference exist in between a bottom surface of the first portion and a bottom surface of the second portion; a first gate oxide layer located on the substrate of the first voltage area, wherein a top surface of the STI structure, a top surface of the first gate oxide layer and a top surface of the substrate in the second voltage area are substantially coplanar to generate a coplanar surface; a second gate oxide layer located on the first gate oxide layer; and a first gate structure located on the second gate oxide layer in the first voltage area.
 2. The semiconductor device according to claim 1, wherein the second gate oxide layer located on the substrate in the second voltage area, and the semiconductor device further comprises: a second gate structure located on the second gate oxide layer in the second voltage area.
 3. The semiconductor device according to claim 1, wherein a bottom surface of the first gate oxide layer is located below the coplanar surface.
 4. The semiconductor device according to claim 1, wherein an operating voltage of the first voltage area is from 6V to 8V.
 5. The semiconductor device according to claim 1, wherein an operating voltage of the second voltage area is from 0.9V to 1V.
 6. The semiconductor device according to claim 1, wherein a material of the second gate oxide layer is HfO₂.
 7. The semiconductor device according to claim 2, wherein a material of the second gate oxide layer is HfO₂.
 8. The semiconductor device according to claim 1, wherein a recess exists in the first voltage area, the recess has a recess depth, and the recess depth and the step height difference correspondingly have the same height.
 9. The semiconductor device according to claim 8, wherein the second voltage area has no recess, such that a top surface of the substrate in the second voltage area is higher than a top surface of the substrate exposed by the recess.
 10. A semiconductor device, comprising: a substrate, having a first voltage area and a second voltage area, wherein a recess exists in the substrate in the first voltage area, such that a top surface of the substrate exposed by the recess is lower than a top surface of the substrate in the second voltage area, and the recess has a recess depth; a shallow trench isolation (STI) structure located in the substrate in between the first voltage area and the second voltage area, wherein a first portion of the STI structure is located in the first voltage area, a second portion of the STI structure is located in the second voltage area.
 11. The semiconductor device according to claim 10, further comprising: a first gate oxide layer located on the substrate of the first voltage area in the recess, wherein a top surface of the STI structure, a top surface of the first gate oxide layer and the top surface of the substrate in the second voltage area are substantially coplanar to generate a coplanar surface; a second gate oxide layer located on the first gate oxide layer; and a first gate structure located on the second gate oxide layer in the first voltage area.
 12. The semiconductor device according to claim 10, further comprising: a second gate oxide layer located on the substrate in the second voltage area; and a second gate structure located on the second gate oxide layer in the second voltage area.
 13. The semiconductor device according to claim 11, wherein a bottom surface of the first gate oxide layer is located below the coplanar surface.
 14. The semiconductor device according to claim 10, wherein an operating voltage of the first voltage area is from 6V to 8V.
 15. The semiconductor device according to claim 10, wherein an operating voltage of the second voltage area is from 0.9V to 1V.
 16. The semiconductor device according to claim 11, wherein a material of the second gate oxide layer is HfO2.
 17. The semiconductor device according to claim 12, wherein a material of the second gate oxide layer is HfO2.
 18. The semiconductor device according to claim 10, wherein a step height difference exists in between a bottom surface of the first portion and a bottom surface of the second portion.
 19. The semiconductor device according to claim 18, wherein the recess depth and the step height difference correspondingly have the same height. 